Optical lithography is still the principal method today in the manufacturing of integrated circuits with a line width down to about one micrometer. The degradation of its replicated pattern of narrower line width arises from two effects: (1) diffraction at the line edges, and (2) interference due to reflections. Recently, work on X-ray lithography indicates that these two problems are alleviated in the X-ray system. Various schemes are being investigated in several laboratories for submicron applications. It turned out that the range of the internal electrons created by the X-ray may become a limiting factor in the lithographic resolution.
One of the attractive advantages of the electron beam lithography rather than the optical and X-ray lithographies is the ability to produce a two-dimensional pattern without utilizing an intricate mask. With a resist layer of 0.5 micrometer thick, structures of submicron linewidth have already been produced in the electron beam system. In the technology of fine-line electron beam lithography, space charge and electron scatterings have been the fundamental factors that limit line resolution. Improvement in pattern resolution of microfabrications on silicon substrates has been prevented by electron scattering and back scattering.
Study of energy dissipation in thin polymer films under electron beam bombardment has shown that even with an electron beam of negligible diameter which is incident upon a resist surface, the profile diameter that will be developed is dominantly determined by multiple electron scattering as illustrated in FIG. 1. It is well known in the art that a 10 KeV electron beam, having a width of about 1.1 micrometer, will have an electron range of about 2.1 micrometers. Although the proportion of beam width to electron range changes with different materials, an increase in range will generally produce an increase in beam width and conversely a decrease in electron range will mean a decrease in beam width. However, it is well known in the prior art that it is very difficult to produce very small electron beam diameters with low values of voltage and that moderate values are required to effectively break the atomic bonding of a resist layer.